Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment, as examples. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductive layers of material over a semiconductor substrate, and patterning the various material layers using lithography to form circuit components and elements thereon.
Some semiconductor devices comprise memory devices that are used to store information. A recent development in semiconductor memory devices is magnetic random access memory (MRAM) devices recording “1” or “0” through a magnetic polarity of ferromagnetic layer. However, the magnetic polarity of the ferromagnetic layer may be effected by external magnetic field. For example, MRAM may be carried or position next to a source emitting magnetic field, which may cause data lost. Thus, at end of wafer processing, performance of the MRAM is checked during wafer acceptance test (WAT) as to ensure a proper operation of the MRAM.
One of the ways to test the data retention of the MRAM devices during the production phase is to utilize the thermal property of the ferromagnetic materials. Wafer having MRAM devices are placed into an oven and bake for a long period of time. Data stored in the MRAM devices are compared before and after the baking process as to identify reversal of magnetic polarity in some of the MRAM cells.